Parts, such as aircraft engine airfoils may be fabricated with hollow interiors to save weight or material. Mechanical performance requires that a specified minimum wall thickness, which may not be constant, be maintained between the outside of the finished part and the interior cavity. However, the part manufacturing process may make it impossible to maintain the exact cavity shape or position. In such cases, the in-process part may be made overly thick and then the outside finish-machined to net shape. The process of positioning the finish machining envelop is complicated, however, when the interior cavity cannot be accessed because of its location or shape or because it is filled with other structural material. Prior art positioning techniques require the use of x-ray tomography to find wall thickness at various sections of a part. The position of the machining envelope is then determined manually using the x-ray pictures. This method is both expensive, slow, and prone to inaccuracy. What is needed therefore is an improved method for positioning the machining envelope which is quicker, more accurate and more automated.